U.S. patent application number 15/115930 was filed with the patent office on 2017-06-15 for negative electrode active material, negative electrode and battery.
The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Tatsuo NAGATA, Noriyuki NEGI, Sukeyoshi YAMAMOTO.
Application Number | 20170170471 15/115930 |
Document ID | / |
Family ID | 54008601 |
Filed Date | 2017-06-15 |
United States Patent
Application |
20170170471 |
Kind Code |
A1 |
YAMAMOTO; Sukeyoshi ; et
al. |
June 15, 2017 |
NEGATIVE ELECTRODE ACTIVE MATERIAL, NEGATIVE ELECTRODE AND
BATTERY
Abstract
Provided is a negative electrode active material that can
improve the discharge capacity per volume and charge-discharge
cycle characteristics. The negative electrode active material of
the present embodiment includes a powder material and an oxide
layer. The powder material contains an alloy phase which undergoes
thermoelastic diffusionless transformation when releasing metal
ions or occluding the metal ions. The oxide layer is formed on the
surface of the powder material, and has a thickness of not more
than 10 nm.
Inventors: |
YAMAMOTO; Sukeyoshi;
(Nishinomiya-shi, Hyogo, JP) ; NEGI; Noriyuki;
(Kisarazu-shi, Chiba, JP) ; NAGATA; Tatsuo;
(Ikeda-shi, Osaka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
54008601 |
Appl. No.: |
15/115930 |
Filed: |
February 25, 2015 |
PCT Filed: |
February 25, 2015 |
PCT NO: |
PCT/JP2015/000958 |
371 Date: |
August 2, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/0404 20130101;
H01M 10/0525 20130101; H01M 4/134 20130101; C22C 9/02 20130101;
H01M 2004/027 20130101; H01M 4/043 20130101; H01M 4/0471 20130101;
Y02E 60/10 20130101; H01M 4/364 20130101; H01M 4/622 20130101; H01M
4/625 20130101; H01M 4/387 20130101; H01M 4/38 20130101 |
International
Class: |
H01M 4/38 20060101
H01M004/38; C22C 9/02 20060101 C22C009/02; H01M 4/62 20060101
H01M004/62; H01M 4/04 20060101 H01M004/04; H01M 10/0525 20060101
H01M010/0525; H01M 4/134 20060101 H01M004/134 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 2014 |
JP |
2014-034272 |
Claims
1-12. (canceled)
13. A negative electrode active material, comprising: a powder
material containing an alloy phase which undergoes thermoelastic
diffusionless transformation when releasing metal ions or occluding
the metal ions; and an oxide layer which is formed on the surface
of the powder material, and has a thickness of not more than 10
nm.
14. The negative electrode active material according to claim 13,
wherein an atomic content of oxygen is not more than 70 at % in the
oxide layer.
15. The negative electrode active material according to claim 13,
wherein the alloy phase undergoes thermoelastic diffusionless
transformation when occluding the metal ions, and undergoes reverse
transformation when releasing the metal ions.
16. The negative electrode active material according to claim 14,
wherein the alloy phase undergoes thermoelastic diffusionless
transformation when occluding the metal ions, and undergoes reverse
transformation when releasing the metal ions.
17. The negative electrode active material according to claim 15,
wherein the alloy phase after the thermoelastic diffusionless
transformation contains a crystal structure which is 2H in Ramsdell
notation, and the alloy phase after the reverse transformation
contains a crystal structure which is DO.sub.3 in Strukturbericht
notation.
18. The negative electrode active material according to claim 16,
wherein the alloy phase after the thermoelastic diffusionless
transformation contains a crystal structure which is 2H in Ramsdell
notation, and the alloy phase after the reverse transformation
contains a crystal structure which is DO.sub.3 in Strukturbericht
notation.
19. The negative electrode active material according to claim 15,
wherein the alloy phase contains Cu and Sn.
20. The negative electrode active material according to claim 16,
wherein the alloy phase contains Cu and Sn.
21. The negative electrode active material according to claim 17,
wherein the alloy phase contains Cu and Sn.
22. The negative electrode active material according to claim 18,
wherein the alloy phase contains Cu and Sn.
23. The negative electrode active material according to claim 20,
wherein the alloy phase contains 10 to 20 at % or 21 to 27 at % of
Sn, with the balance being Cu and impurities.
24. The negative electrode active material according to claim 22,
wherein the alloy phase contains 10 to 20 at % or 21 to 27 at % of
Sn, with the balance being Cu and impurities.
25. The negative electrode active material according to claim 20,
wherein the alloy phase contains, in place of a part of Cu, one or
more selected from the group consisting of Ti, V, Cr, Mn, Fe, Co,
Ni, Zn, Al, Si, B, and C.
26. The negative electrode active material according to claim 22,
wherein the alloy phase contains, in place of a part of Cu, one or
more selected from the group consisting of Ti, V, Cr, Mn, Fe, Co,
Ni, Zn, Al, Si, B, and C.
27. The negative electrode active material according to claim 25,
wherein the alloy phase contains: Sn: 10 to 35 at %; and one or
more selected from the group consisting of Ti: 9.0 at % or less, V:
49.0 at % or less, Cr 49.0 at % or less, Mn: 9.0 at % or less, Fe:
49.0 at % or less, Co: 49.0 at % or less, Ni: 9.0 at % or less, Zn:
29.0 at % or less, Al: 49.0 at % or less, Si: 49.0 at % or less, B:
5.0 at % or less, and C: 5.0 at % or less, with the balance being
Cu and impurities.
28. The negative electrode active material according to claim 26,
wherein the alloy phase contains: Sn: 10 to 35 at %; and one or
more selected from the group consisting of Ti: 9.0 at % or less, V:
49.0 at % or less, Cr: 49.0 at % or less, Mn: 9.0 at % or less, Fe:
49.0 at % or less, Co: 49.0 at % or less, Ni: 9.0 at % or less, Zn:
29.0 at % or less, Al: 49.0 at % or less, Si: 49.0 at % or less, B:
5.0 at % or less, and C: 5.0 at % or less, with the balance being
Cu and impurities.
29. A negative electrode active material, comprising: powder
material containing an alloy phase which contains 10 to 20 at % or
21 to 27 at % of Sn, with the balance being Cu and impurities; and
an oxide layer which is formed on the surface of the powder
material, and has a thickness of not more than 10 nm.
30. A negative electrode active material comprising: powder
material containing an alloy phase which contains: Sn: 10 to 35 at
%; and one or more selected from the group consisting of Ti: 9.0 at
% or less, V: 49.0 at % or less, Cr 49.0 at % or less, Mn: 9.0 at %
or less, Fe: 49.0 at % or less, Co: 49.0 at % or less, Ni: 9.0 at %
or less, Zn: 29.0 at % or less, Al: 49.0 at % or less, Si: 49.0 at
% or less, B: 5.0 at % or less, and C: 5.0 at % or less, with the
balance being Cu and impurities; and an oxide layer which is formed
on the surface of the powder material, and has a thickness of not
more than 10 nm.
31. A negative electrode, comprising the negative electrode active
material according to claim 13.
32. A battery, comprising the negative electrode according to claim
31.
Description
TECHNICAL FIELD
[0001] The present invention relates to an electrode active
material, and more particularly to a negative electrode active
material.
BACKGROUND ART
[0002] Recently, small electronic appliances such as home video
cameras, note PCs, and smart phones have become widespread, and
attaining higher capacity and longer service life of batteries has
become a technical problem.
[0003] Given that hybrid vehicles, plug-in hybrid vehicles, and
electric vehicles will be further spread, size reduction of
batteries is also a technical problem.
[0004] At present, graphite-based negative electrode active
materials are utilized for lithium ion batteries. However,
graphite-based negative electrode active materials have technical
problem as described above.
[0005] Accordingly, alloy-based negative electrode active materials
have gained attention, which have higher capacity than those of the
graphite-based negative electrode active materials. As an
alloy-based negative electrode active material, silicon (Si)-based
negative electrode active materials and tin (Sn)-based negative
electrode active materials are known. To realize a lithium ion
battery having a smaller size and a longer life, various studies
have been conducted on the above described alloy-based negative
electrode active materials.
[0006] However, an alloy-based negative electrode active material
repeatedly undergoes large expansion and contraction in volume at
the time of charging/discharging. For that reason, the capacity of
the alloy-based negative electrode active material is prone to
deteriorate. For example, a volume expansion/contraction ratio of
graphite associated with charging is about 12%. In contrast, the
volume expansion/contraction ratio of Si single substance or Sn
single substance associated with charging is about 400%. For this
reason, if a negative electrode plate of Si single substance or Sn
single substance is repeatedly subjected to charging and
discharging, significant expansion and contraction occur, thereby
causing cracking in negative electrode compound which is applied on
the current collector of the negative electrode plate.
Consequently, the capacity of the negative electrode plate rapidly
decreases. This is chiefly caused by the fact that some of the
active substances are freed due to volume expansion/contraction and
thereby the negative electrode plate loses electron
conductivity.
[0007] US2008/0233479A (Patent Literature 1) proposes a method for
solving the above described problem of an alloy-based negative
electrode active material. To be more specific, the negative
electrode material on Patent Literature 1 includes a Ti--Ni
superelastic alloy, and Si particles formed in the superelastic
alloy. Patent Literature 1 describes that a large
expansion/contraction change of Si particle, which occurs following
occlusion and release of lithium ions, can be suppressed by the
superelastic alloy.
CITATION LIST
Patent Literature
Patent Literature 1: US2008/0233479A
[0008] However, the charge-discharge cycle characteristics of the
secondary battery in Patent Literature 1 may not be sufficiently
improved by the technique disclosed therein. Most of all, it may be
highly difficult to actually produce the negative electrode active
material proposed by Patent Literature 1.
SUMMARY OF INVENTION
[0009] It is an objective of the present invention to provide a
negative electrode active material which can improve the discharge
capacity per volume and/or charge-discharge cycle characteristics
thereof.
[0010] The negative electrode active material of the present
embodiment includes a powder material and an oxide layer. The
powder material contains an alloy phase which undergoes
thermoelastic diffusionless transformation when releasing metal
ions or occluding metal ions. The oxide layer is formed on the
surface of the powder material, and has a thickness of not more
than 10 nm.
[0011] The negative electrode active material of the present
embodiment can improve the discharge capacity per volume and the
charge-discharge cycle characteristics.
BRIEF DESCRIPTION OF DRAWINGS
[0012] FIG. 1 is a perspective view of DO.sub.3 structure.
[0013] FIG. 2A is a schematic diagram of DO.sub.3 structure of the
matrix phase of the alloy phase of the present embodiment.
[0014] FIG. 2B is a schematic diagram of 2H structure of .gamma.1'
phase which is a kind of martensite phase.
[0015] FIG. 2C is a schematic diagram of a crystal plane to explain
thermoelastic diffusionless transformation from DO.sub.3 structure
to 2H structure.
[0016] FIG. 2D is a schematic diagram of another crystal plane
different from that of FIG. 2C.
[0017] FIG. 2E is a schematic diagram of another crystal plane
different from those of FIGS. 2C and 2D.
[0018] FIG. 3 is a diagram illustrating an X-ray diffraction
profiles before and after charging-discharging of the alloy phase
of Inventive Example 1 of the present invention, and simulation
results by Rietveld method.
DESCRIPTION OF EMBODIMENTS
[0019] Hereinafter, with reference to the drawings, embodiments of
the present invention will be described in detail. Like parts or
corresponding parts in the drawings are given a like reference
symbol and description thereof will not be repeated.
[0020] The negative electrode active material according to the
present embodiment includes a powder material and an oxide layer.
The powder material contains an alloy phase. The alloy phase
undergoes thermoelastic diffusionless transformation when releasing
or occluding metal ions. The powder material includes an oxide
layer on the surface thereof. The thickness of the oxide layer is
not more than 10 nm.
[0021] A "negative electrode active material" referred herein is
preferably a negative electrode active material for nonaqueous
electrolyte secondary batteries. A "thermoelastic diffusionless
transformation" referred herein is so-called thermoelastic
martensitic transformation. A "metal ion" refers to, for example, a
lithium ion, magnesium ion, sodium ion, and the like. A preferable
metal ion is lithium ion.
[0022] This negative electrode active material may contain other
phases different from the above described alloy phases. The other
phases include, for example, a silicon (Si) phase, a tin (Sn)
phase, other alloy phases (alloy phases which do not undergo
thermoelastic diffusionless transformation) excepting the above
described alloy phases, and the like.
[0023] Preferably, the above described alloy phases are main
components (main phases) of the negative electrode active material.
"Main component" refers to a component which occupies not less than
50% by volume. The alloy phase may contain impurities to the extent
that the spirit of the present invention is unimpaired. However,
the impurities are contained preferably as little as possible.
[0024] A negative electrode formed of a negative electrode active
material of the present embodiment has a higher volumetric
discharge capacity (discharge capacity per volume) than that of a
negative electrode made of graphite, when used in a nonaqueous
electrolyte secondary battery. Further, a nonaqueous electrolyte
secondary battery using a negative electrode containing a negative
electrode active material of the present embodiment has a higher
capacity retention ratio than one using a conventional alloy-based
negative electrode. Therefore, the negative electrode active
material can sufficiently improve the charge-discharge cycle
characteristics of the nonaqueous electrolyte secondary
battery.
[0025] A possible reason why the capacity retention ratio is high
is that strain, due to expansion/contraction that occurs at the
time of charging/discharging, is relaxed by thermoelastic
diffusionless transformation. Moreover, the fact that the powder
material includes an oxide layer on the surface thereof, and the
thickness of the oxide layer is not more than 10 nm may also
contribute to a high capacity retention ratio. Details thereof will
be described later.
[0026] The alloy phase may be of any one of the following types 1
to 4.
[0027] The alloy phase of type 1 undergoes thermoelastic
diffusionless transformation when occluding metal ions, and
undergoes reverse transformation when releasing metal ions. In this
case, the alloy phase is a matrix phase in a normal state.
[0028] The alloy phase of type 2 undergoes reverse transformation
when occluding metal ions, and undergoes thermoelastic
diffusionless transformation when releasing metal ions. In this
case, the alloy phase is a martensite phase in a normal state.
[0029] The alloy phase of type 3 undergoes supplemental deformation
(slip deformation or twin deformation) when occluding metal ions,
and returns to the original martensite phase when releasing metal
ions. In this case, the alloy phase is a martensite phase in a
normal state.
[0030] The alloy phase of type 4 transforms from a martensite phase
to another martensite phase when occluding metal ions, and returns
to the original martensite phase when releasing metal ions. In this
case, the alloy phase is a martensite phase in a normal state.
[0031] In the case of the alloy phase of type 1, preferably, the
crystal structure of the alloy phase after thermoelastic
diffusionless transformation is either of 2H, 3R, 6R, 9R, 18R, M2H,
M3R, M6R, M9R, and M18R in Ramsdell notation, and the crystal
structure of the alloy phase after reverse transformation is
DO.sub.3 in Strukturbericht notation. More preferably, the crystal
structure of the alloy phase after thermoelastic diffusionless
transformation is the above described 2H, and the crystal structure
of the alloy phase after reverse transformation is the above
described DO.sub.3.
[0032] In the case of the alloy phase of type 1, preferably, the
alloy phase contains Cu and Sn, and also contains the above
described 2H structure after thermoelastic diffusionless
transformation, and the above described DO.sub.3 structure after
reverse transformation.
[0033] The above described alloy phase may contain one or more
selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni,
Zn, Al, Si, B, and C, and Sn, with balance being Cu and
impurities.
[0034] The above described alloy phase may also contain one or more
selected from the group consisting of .delta. phase of F-Cell
structure, .epsilon. phase of 2H structure, .eta.' phase of
monoclinic crystal, and a phase having DO.sub.3 structure, each
including site deficiency.
[0035] All of these .delta. phase, .epsilon. phase, .eta.' phase,
and phase having DO.sub.3 structure, each including site deficiency
form a storage site and a diffusion site of metal ions (Li ions,
etc.) in the negative electrode active material. Thereby, the
volumetric discharge capacity and the cycle characteristics of the
negative electrode active material are further improved.
[0036] In the above described negative electrode active material, a
volume expansion ratio or volume contraction ratio of a unit cell
of the above described alloy phase before and after the phase
transformation is preferably not more than 20%, and more preferably
not more than 10%. The volume expansion ratio of unit cell is
defined by the following Formula (1), and the volume contraction
ratio of unit cell is defined by the following Formula (2).
(Volume expansion ratio of unit cell)=[(volume of unit cell when
metal ions are occluded)-(volume of unit cell when metal ions are
released)]/(volume of unit cell when metal ions are
released).times.100 (1)
(Volume contraction ratio of unit cell)=[(volume of unit cell when
metal ions are occluded)-(volume of unit cell when metal ions are
released)]/(volume of unit cell when metal ions are
occluded).times.100 (2)
[0037] The volume of unit cell at the time of releasing, which
corresponds to a crystal lattice range of unit cell at the time of
occluding, is substituted into "volume of unit cell when metal ions
are released" in Formulas (1) and (2).
[0038] The above described negative electrode active material can
be used as active material for making up an electrode, particularly
electrode of a nonaqueous electrolyte secondary battery. An example
of the nonaqueous electrolyte secondary battery is a lithium ion
secondary battery.
[0039] Hereinafter, negative electrode active materials according
to the present embodiment will be described in detail.
<Negative Electrode Active Material>
[0040] A negative electrode active material relating to embodiments
of the present invention contains a powder material and an oxide
layer. The powder material contains an alloy phase. The alloy phase
undergoes thermoelastic diffusionless transformation when releasing
or occluding metal ions. The powder material has an oxide layer on
the surface thereof. The thickness of the oxide layer is not more
than 10 nm. The power material contains a plurality of powder
particles. The powder material is substantially composed of a
plurality of powder particles, and more particularly the powder
material is composed of a plurality of powder particles and
impurities. Therefore, the oxide layer is formed on the surface of
the powder particle.
[0041] The oxygen concentration in the oxide layer of the surface
is generally highest at the surface, and exhibits a concentration
gradient decreasing in the depth direction, eventually reaching a
constant value at a certain depth. In the present embodiment, the
oxide layer of the surface refers to a layer having a thickness
corresponding to a half of the depth at which the oxygen
concentration becomes constant, when the alloy powder is subjected
to quantitative analysis of oxygen from the surface in the depth
direction.
[Alloy Phase]
[0042] The alloy phase undergoes thermoelastic diffusionless
transformation when releasing metal ions represented by Li ions, or
occluding the metal ions, as described above. The thermoelastic
diffusionless transformation is also called as thermoelastic
martensitic transformation. Hereinafter, in the present
description, the thermoelastic martensitic transformation is simply
referred to as "M transformation" and the martensite phase as "M
phase". An alloy phase that undergoes M transformation when
occluding or releasing metal ions is also referred to as a
"specific alloy phase".
[0043] The specific alloy phase is dominantly made up of at least
one of M phase and a matrix phase. The specific alloy phase repeats
occlusion/release of metal ions at the time of
charging/discharging. Then, the specific alloy phase undergoes M
transformation, reverse transformation, supplemental deformation,
etc. in response to occlusion and release of metal ions. These
transformation behaviors mitigate strain which is caused by
expansion and contraction of the alloy phase when occluding and
releasing metal ions.
[0044] The specific alloy phase may be any one of the above
described types 1 to 4. Preferably, the specific alloy phase is of
type 1. That is, the specific alloy phase preferably undergoes M
transformation when occluding metal ions, and undergoes reverse
transformation when releasing metal ions.
[0045] The crystal structure of the specific alloy phase is not
specifically limited. If the alloy phase is of type 1, and the
crystal structure of the specific alloy phase (that is, a matrix
phase) after reverse transformation is .beta..sub.1 phase (DO.sub.3
structure), the crystal structure of the specific alloy phase (that
is, M phase) after M transformation is, for example, .beta..sub.1'
phase (M18R.sub.1 structure of monoclinic crystal or 18R.sub.1
structure of orthorhombic crystal), .gamma..sub.1' phase (M2H
structure of monoclinic crystal or 2H structure of orthorhombic
crystal), .beta..sub.1'' phase (M18R.sub.2 structure of monoclinic
crystal or 18R.sub.2 structure of orthorhombic crystal),
.alpha..sub.1' phase (M6R structure of monoclinic crystal or 6R
structure of orthorhombic crystal), and the like.
[0046] If the crystal structure of the matrix phase of the specific
alloy phase is 02 phase (B2 structure), the crystal structure of M
phase of the specific alloy phase is, for example, .beta..sub.2'
phase (M9R structure of monoclinic crystal or 9R structure of
orthorhombic crystal), .gamma..sub.2' phase (M2H structure of
monoclinic crystal or 2H structure of orthorhombic crystal), and
.alpha..sub.2' phase (M3R structure of monoclinic crystal or 3R
structure of orthorhombic crystal).
[0047] If the matrix phase of the alloy phase has a face-centered
cubic lattice, the crystal structure of M phase of the alloy phase
has, for example, a face-centered tetragonal lattice, and a
body-centered tetragonal lattice.
[0048] Such symbols as the above described 2H, 3R, 6R, 9R, 18R,
M2H, M3R, M6R, M9R, and M18R are used as the method of denoting
crystal structures of a layered construction according to
Ramsdell's classification. The symbols H and R mean that respective
symmetries in the direction perpendicular to the lamination plane
are hexagonal symmetry and rhombohedral symmetry. If there is no M
appended at the beginning, it means that the crystal structure is
an orthorhombic crystal. If there is M appended at the beginning,
it means that the crystal structure is a monoclinic crystal. Even
if same classification symbols are used, there are cases in which
distinction is made by the difference in the order of the layers.
For example, since .beta..sub.1' phase and .beta..sub.1'' phase,
which are two kinds of M phase, have a different layered
construction, there are cases in which they are distinguished by
being denoted as 18R.sub.1 and 18R.sub.2, or M18R.sub.1 and
M18R.sub.2 etc., respectively.
[0049] In General, M transformation and reverse transformation in
normal shape memory effects and pseudoelastic effects often involve
volume contraction or volume expansion. When a negative electrode
active material relating to the present embodiment
electrochemically releases or occludes metal ions (for example,
lithium ions), it is considered that the crystal structure often
changes in consistent with the phenomena of volume contraction or
volume expansion in the direction of respective transformation.
[0050] However, the negative electrode active material according to
the present embodiment will not be particularly limited by such
restriction. When M transformation or reverse transformation occurs
following occlusion and release of metal ions in the specific alloy
phase, there may be generated other crystal structures than the
crystal structure that appears at the time of ordinary shape memory
effects and pseudoelastic effects.
[0051] When the specific alloy phase is of type 3, the specific
alloy phase undergoes slip deformation or twin deformation
following occlusion or release of metal ions. In slip deformation,
since dislocation is introduced as the lattice defect, reversible
deformation is difficult. Therefore, when the specific alloy phase
is of type 3, it is preferable that twin deformation dominantly
occurs.
[Chemical Composition of Specific Alloy Phase]
[0052] The chemical composition of a negative electrode active
material containing the above described specific alloy phase will
not be particularly limited provided that the crystal structure at
the time of M transformation and reverse transformation contains
the above described crystal structures.
[0053] When the specific alloy phase is of type 1, the chemical
composition of the negative electrode active material containing
the specific alloy phase contains, for example, Cu (copper) and Sn
(tin).
[0054] When the specific alloy phase is of type 1, preferably, the
crystal structure of the specific alloy phase after reverse
transformation caused by discharge of metal ions is DO.sub.3
structure, and the crystal structure of the specific alloy phase
after M transformation caused by occlusion of metal ions is 2H
structure.
[0055] Preferably, the chemical composition of a specific alloy
phase contains Sn, with the balance being Cu and impurities. More
preferably, the specific alloy phase contains 10 to 20 at % or 21
to 27 at % of Sn, with the balance being Cu and impurities, wherein
the specific alloy phase contains 2H structure after M
transformation, and DO.sub.3 structure after reverse
transformation. A more preferable Sn content in the specific alloy
phase is 13 to 16 at %, 18.5 to 20 at %, or 21 to 27 at %.
[0056] The chemical composition of a specific alloy phase may
contain one or more selected from the group consisting of Ti, V,
Cr, Mn, Fe, Co, Ni, Zn, Al, Si, B, and C, and Sn, with the balance
being Cu and impurities.
[0057] Preferably, the chemical composition of the specific alloy
phase in this case contains: Sn: 10 to 35 at %, and one or more
selected from the group consisting of Ti: 9.0 at % or less, V: 49.0
at % or less, Cr: 49.0 at % or less, Mn: 9.0 at % or less, Fe: 49.0
at % or less, Co: 49.0 at % or less, Ni: 9.0 at % or less, Zn: 29.0
at % or less, Al: 49.0 at % or less, Si: 49.0 at % or less, B: 5.0
at % or less, and C: 5.0 at % or less, with the balance being Cu
and impurities. The above described Ti, V, Cr, Mn, Fe, Co, Ni, Zn,
Al, Si, B and C are optional elements.
[0058] A preferable upper limit of Ti content is 9.0 at % as
described above. The upper limit of Ti content is more preferably
6.0 at %, and further preferably 5.0 at %. A lower limit of Ti
content is preferably 0.1 at %, more preferably 0.5 at %, and
further preferably 1.0 at %.
[0059] A preferable upper limit of V content is 49.0 at % as
described above. The upper limit of V content is more preferably
30.0 at %, further preferably 15.0 at %, and furthermore preferably
10.0 at %. A lower limit of V content is preferably 0.1 at %, more
preferably 0.5 at %, and further preferably 1.0 at %.
[0060] A preferable upper limit of Cr content is 49.0 at % as
described above. The upper limit of Cr content is more preferably
30.0 at %, further preferably 15.0 at %, and furthermore preferably
10.0 at %. A lower limit of Cr content is preferably 0.1 at %, more
preferably 0.5 at %, and further preferably 1.0 at %.
[0061] A preferable upper limit of Mn content is 9.0 at % as
described above. The upper limit of Mn content is more preferably
6.0 at %, and further preferably 5.0 at %. A lower limit of Mn
content is preferably 0.1 at %, more preferably 0.5 at %, and
further preferably 1.0 at %.
[0062] A preferable upper limit of Fe content is 49.0 at % as
described above. The upper limit of Fe content is more preferably
30.0 at %, further preferably 15.0 at %, and furthermore preferably
10.0 at %. A lower limit of Fe content is preferably 0.1 at %, more
preferably 0.5 at %, and further preferably 1.0 at %.
[0063] A preferable upper limit of Co content is 49.0 at % as
described above. The upper limit of Co content is more preferably
30.0 at %, further preferably 15.0 at %, and furthermore preferably
10.0 at %. A lower limit of Co content is preferably 0.1 at %, more
preferably 0.5 at %, and further preferably 1.0 at %.
[0064] A preferable upper limit of Ni content is 9.0 at % as
described above. The upper limit of Ni content is more preferably
5.0 at %, and further preferably 2.0 at %. A lower limit of Ni
content is preferably 0.1 at %, more preferably 0.5 at %, and
further preferably 1.0 at %.
[0065] A preferable upper limit of Zn content is 29.0 at % as
described above. The upper limit of Zn content is more preferably
27.0 at %, and further preferably 25.0 at %. A lower limit of Zn
content is preferably 0.1 at %, more preferably 0.5 at %, and
further preferably 1.0 at %.
[0066] A preferable upper limit of Al content is 49.0 at % as
described above. The upper limit of Al content is more preferably
30.0 at %, further preferably 15.0 at %, and furthermore preferably
10.0 at %. A lower limit of Al content is preferably 0.1 at %, more
preferably 0.5 at %, and further preferably 1.0 at %.
[0067] A preferable upper limit of Si content is 49.0 at % as
described above. The upper limit of Si content is more preferably
30.0 at %, further preferably 15.0 at %, and furthermore preferably
10.0 at %. A lower limit of Si content is preferably 0.1 at %, more
preferably 0.5 at %, and further preferably 1.0 at %.
[0068] A preferable upper limit of B content is 5.0 at %. The lower
limit of B content is preferably 0.01 at %, more preferably 0.1 at
%, further preferably 0.5 at %, and furthermore preferably 1.0 at
%.
[0069] A preferable upper limit of C content is 5.0 at %. The lower
limit of C content is preferably 0.01 at %, more preferably 0.1 at
%, further preferably 0.5 at %, and furthermore preferably 1.0 at
%.
[0070] Preferably, the specific alloy phase contains one or more
selected from the group consisting of .delta. phase of F-Cell
structure containing site deficiency, a phase of 2H structure
containing site deficiency, .eta.' phase of monoclinic crystal
containing site deficiency, and a phase having DO.sub.3 structure
containing site deficiency. Hereinafter, these .delta. phase,
.epsilon. phase, .eta.' phase, and phase having DO.sub.3 structure,
each containing site deficiency, are also referred to as "site
deficient phase". Here, "site deficiency" means a state of a
crystal structure in which occupancy factor is less than 1 in a
specific atomic site.
[0071] These site deficient phases include a plurality of site
deficiencies in the crystal structure. These site deficiencies
function as a storage site or a diffusion site of metal ions (such
as Li ions). Therefore, if a powder material contains an alloy
phase which becomes 2H structure after M transformation and becomes
DO.sub.3 structure after reverse transformation, and at least one
phase among the above described site deficient phases, the
volumetric discharge capacity and the cycle characteristics of the
negative electrode active material are further improved.
[0072] The chemical composition of a specific alloy phase may
further contain a Group 2 element and/or rare earth metal (REM) for
the purpose of increasing discharge capacity. The Group 2 elements
include, for example, magnesium (Mg) calcium (Ca) and the like.
REMs include, for example, lanthanum (La), cerium (Ce),
praseodymium (Pr), neodymium (Nd) and the like.
[0073] If a specific alloy phase contains a Group 2 element and/or
REM, the powder material becomes brittle. Therefore, in the
production process of the electrode, a bulk material or an ingot
made of the powder material is easy to be pulverized, making it
easy to produce an electrode.
[0074] The powder material may be made up of the above described
specific alloy phase, or may contain the above described specific
alloy phase and another active material phase which is metal
ion-active. Another active material phase includes, for example, a
tin (Sn) phase, a silicon (Si) phase, an aluminum (Al) phase, a
Co--Sn alloy phase, a Cu.sub.6Sn.sub.5 compound phase (.eta.' phase
or .eta. phase) and the like.
[Volume Expansion Ratio and Volume Contraction Ratio of Specific
Alloy Phase]
[0075] When the above described specific alloy phase undergoes M
transformation or reverse transformation along with occlusion and
release of metal ions, preferable volume expansion/contraction
ratio of unit cell of the specific alloy phase is not more than
20%. In this case, it is possible to sufficiently relax the strain
due to a volume change which occurs following occlusion and release
of metal ions. The volume expansion/contraction ratio of unit cell
of the specific alloy phase is more preferably not more than 10%,
and further preferably not more than 5%.
[0076] The volume expansion/contraction ratio of the specific alloy
phase can be measured by an in-situ X-ray diffraction during
charging/discharging. To be more specific, an electrode plate of
negative electrode active material, a separator, a counter
electrode lithium, and electrolytic solution are placed and sealed
in a dedicated charge/discharge cell equipped with a window made of
beryllium which transmits X-ray, within a glove box in pure argon
gas atmosphere in which moisture is controlled such that due point
is not more than -80.degree. C. Then, this charge/discharge cell is
mounted onto the X-ray diffraction apparatus. After mounting, an
X-ray diffraction profile of the specific alloy phase is obtained
in each of an initially charged state and an initially discharged
state in the course of charging and discharging. From this X-ray
diffraction profile, a lattice constant of the specific alloy phase
is found. From the lattice constant, it is possible to calculate
the volume change ratio in consideration of crystal lattice
correspondence of the specific alloy phase.
[0077] When the shape of X-ray diffraction profile changes due to
full width at half maximum etc. in the charge-discharge cycling
process, analysis is performed after repeating charging and
discharging 5 to 20 times as needed. Then, an average value of
volume change ratio is found from a plurality of X-ray diffraction
profiles having high reliability.
[Analysis Method of Crystal Structure of Alloy Phase Contained by
Negative Electrode Active Material]
[0078] (1) The crystal structure of the phase (including an alloy
phase) contained in the negative electrode active material can be
analyzed by Rietveld method based on the X-ray diffraction profile
obtained by using an X-ray diffraction apparatus. To be more
specific, the crystal structure is analyzed by the following
method.
[0079] For a negative electrode active material before use for a
negative electrode, X-ray diffraction measurement is performed on
the negative electrode active material to obtain measured data of
X-ray diffraction profile. Based on the obtained X-ray diffraction
profile (measured data), the configuration of phases in the
negative electrode active material is analyzed by Rietveld method.
For the analysis by Rietveld method, either of "RIETAN2000"
(program name) or "RIETAN-FP" (program name) which are
general-purpose analysis software is used.
[0080] (2) The crystal structure of a negative electrode active
material in a negative electrode before charging in a battery is
determined by the same method as that in (1). To be more specific,
the battery, which is in an uncharged state, is disassembled within
the glove box in argon atmosphere, and the negative electrode is
taken out from the battery. The negative electrode taken out is
enclosed with Myler foil. Thereafter, the perimeter of the Myler
foil is sealed by a thermocompression bonding machine. Then, the
negative electrode sealed by the Myler foil is taken out of the
glove box.
[0081] Next, a measurement sample is fabricated by bonding the
negative electrode to a reflection-free sample plate (a plate of a
silicon single crystal which is cut out such that a specific
crystal plane is in parallel with the measurement plane) with hair
spray. The measurement sample is mounted onto the X-ray diffraction
apparatus and X-ray diffraction measurement of the measurement
sample is performed to obtain an X-ray diffraction profile. Based
on the obtained X-ray diffraction profile, the crystal structure of
the negative electrode active material in the negative electrode is
determined by Rietveld method.
[0082] (3) Crystal structures of the negative electrode active
material in the negative electrode after charging one to multiple
times and after discharging one to multiple times are determined by
the same method as that in (2).
[0083] To be more specific, the battery is fully charged in a
charging/discharging test apparatus. The fully charged battery is
disassembled in the glove box, and a measurement sample is
fabricated by the same method as that in (2). The measurement
sample is mounted onto the X-ray diffraction apparatus and X-ray
diffraction measurement is performed.
[0084] Moreover, the battery is fully discharged, and the fully
discharged battery is disassembled in the glove box and a
measurement sample is fabricated by the same method as that in (2)
to perform X-ray diffraction measurement.
[Making Thickness of Surface Oxide Layer not More than 10 nm]
[0085] The powder material has an oxide layer on the surface
thereof. The thickness of the oxide layer is not more than 10 nm.
Making the thickness of the oxide layer not more than 10 nm will
increase the capacity retention ratio and also improve the
charge-discharge cycle characteristics. A possible reason of this
is considered as follows.
[0086] In an alloy-based active material, a film made from a
combination of a metal ion (Li, etc.), a constituting element of an
alloy phase, and oxygen (for example, Li--Sn--O combination) is
likely to be formed on the surface of the material at the time of
charging and discharging. This film inhibits the movement of the Li
ion associated with charging/discharging. Hereinafter, such a film
is referred to as a "reaction inhibiting film". As the thickness of
such reaction inhibiting film increases, the degree of inhibiting
the occlusion and release of Li etc. increases. For that reason,
the discharge capacity remarkably decreases, and the cycle life
decreases. In the negative electrode active material of the present
embodiment, the thickness of the oxide layer on the surface of the
material is made not more than 10 nm to suppress the growth of such
reaction inhibiting film. Since the thickness of the oxide layer is
small, the supply amount of oxygen to the reaction inhibiting film
is small, thus suppressing the growth of the reaction inhibiting
film. The thickness of the oxide layer is preferably not more than
5 nm.
[0087] The oxygen concentration in the oxide layer of the surface
is generally highest at the surface, and exhibits a concentration
gradient decreasing in the depth direction, eventually reaching a
constant value at a certain depth. In the present description, the
oxide layer of the surface refers to a layer having a thickness
corresponding to a half of the depth at which the oxygen
concentration becomes constant when the alloy powder is subjected
to quantitative analysis of oxygen from the surface in the depth
direction. The thickness of the oxide layer of the surface can be
measured by, for example, Auger electron spectroscopy. In this
case, first, a relatively flat region of power particles of a size
of several tens .mu.m is selected in an scanning electron
microscope mode, and an electron beam is reduced to detect the
intensity of Auger electrons resulted from oxygen from a region of
a several .mu.m square. An intensity profile in the depth direction
can be obtained by subjecting a specimen to argon ion sputtering to
remove a fixed thickness in the depth direction by grinding, and
then to measurement by Auger electron spectroscopy, and repeating
these processes. From the obtained profile, the thickness of layer
of the oxide film is determined as the distance to a position
corresponding to a half of the intensity at the outer most
surface.
[Making Oxygen Concentration in Surface Oxide Layer not More than
70 at %]
[0088] The oxygen concentration in the oxide layer of material
surface is preferably not more than 70 at %. This makes it possible
to further suppress the formation of a reaction inhibiting film.
This makes it possible to further suppress the decrease in the
discharge capacity due to repetition of charging and discharging.
The oxygen concentration is more preferably not more than 50 at %.
A lower limit of the oxygen concentration is, for example, 10 at
%.
<Production Method of Negative Electrode Active Material>
[0089] A production method of a negative electrode active material,
which contains the above-described specific alloy phase, and has an
oxide layer of a specific thickness, will be described.
[0090] Molten metal which is the raw material of the powder
material containing the specific alloy phase is produced. For
example, molten metal having the above described chemical
composition is produced. The molten metal is produced by melting
starting material by an ordinary melting method such as arc melting
or resistance heating melting. Next, an ingot (bulk alloy) is
produced by an ingot casting method by using the molten metal.
[0091] Alternatively, a thin cast piece or particle is produced,
preferably by subjecting the molten metal to rapid solidification.
This method is called a rapid solidification method. Examples of
the rapid solidification method include a strip casting method, a
melt-spinning method for producing ribbons, a gas atomization
method, a melt spinning method for producing fibers, a water
atomization method, an oil atomization method, and the like.
[0092] The bulk alloy (ingot) obtained by melting is (1) cut, (2)
coarsely crushed by a hammer mill etc., or (3) finely pulverized
mechanically by a ball mill, an attritor, a disc mill, a jet mill,
a pin mill, and the like to produce the powder material which is
adjusted into a necessary particle size. When the bulk alloy has
ductility and ordinary pulverization is difficult, the bulk alloy
may be subjected to cutting and pulverization by a grinder disc,
which is embedded with diamond abrasive particles, and the like.
When M phase due to stress induction is formed in these
pulverization processes, the formation ratio thereof is adjusted as
needed by appropriately combining the alloy design, heat treatment,
and pulverization conditions thereof. When powder generated by an
atomization method can be used as melted or as heat treated, there
may be cases where no pulverization process is particularly needed.
Moreover, when melted material is obtained by a strip casting
method and crushing thereof is difficult due to its ductility, the
melted material is adjusted to have a predetermined size by being
subjected to mechanical cutting such as shearing. Moreover, in such
a case, the melted material may be heat treated in a necessary
stage, to adjust the ratio between M phase and a matrix phase, and
the like.
[0093] When the powder material is heat treated to adjust the
constitution ratio, etc. of the specific alloy phase, the powder
material may be rapidly cooled after being retained at a
predetermined temperature for a predetermined time period in inert
atmosphere as needed. In this occasion, the cooling rate may be
adjusted by selecting a quenching medium such as water, salt water
with ice, oil, and liquid nitrogen according to the size of the
powder material, and setting the quenching medium to a
predetermined temperature. Moreover, immediately after the
quenching, liquid nitrogen sub-zero treatment may be performed.
This liquid nitrogen sub-zero treatment allows preparation of the
constitution ratio of the specific alloy phase, and adjustment of
martensite transformation temperature.
[0094] The thickness of the oxide layer of the surface of the
powder material is made not more than 10 nm, and preferably, for
example at least any of the followings is performed to make the
oxygen concentration in the oxide layer not more than 70 at %.
[0095] In the production process of molten metal, the oxygen
concentration in the atmospheric gas is made not more than 5000
ppm, and preferably not more than 1000 ppm. [0096] The atmosphere
in the process of solidification is made to be a vacuum atmosphere
or an inert gas atmosphere, in which the oxygen content is reduced
as much as possible. [0097] The atmosphere in the pulverization
process is made to be an inert gas atmosphere in which the oxygen
content is reduced as much as possible. [0098] The atmosphere in
the heat treatment process for adjusting the constitution ratio,
etc. of the specific alloy phase is made to be an inert gas
atmosphere, a vacuum atmosphere, or a reducing atmosphere, in which
the oxygen content is reduced as much as possible.
[0099] Through the above described processes, a negative electrode
active material containing the above described plurality of powder
materials is produced.
<Production Method of Negative Electrode>
[0100] A negative electrode using a negative electrode active
material relating to an embodiment of the present invention can be
produced by a method well known to those skilled in the art.
[0101] For example, a binder such as polyvinylidene fluoride
(PVDF), polymethyl methacrylate (PMMA), polytetrafluoroethylene
(PTFE), and styrene-butadiene rubber (SBR) is admixed to powder of
a negative electrode active material of an embodiment of the
present invention, and further carbon material powder such as
natural graphite, artificial graphite, and acetylene black is
admixed thereto to impart sufficient conductivity to the negative
electrode. After being dissolved by adding a solvent such as
N-methylpyrrolidone (NMP), dimethylformamide (DMF) and water, the
binder is stirred well using a homogenizer and glass beads if
necessary, and formed into a slurry. This slurry is applied on an
active substance support member such as a rolled copper foil and an
electrodeposited copper foil and is dried. Thereafter, the dried
product is subjected to pressing. Through the above described
processes, a negative electrode plate is produced.
[0102] The amount of the binder to be admixed is preferably about 5
to 10 mass % from the viewpoint of the mechanical strength and
battery characteristics of the negative electrode. The support
member is not limited to a copper foil. The support member may be,
for example, a foil of other metals such as stainless steel and
nickel, a net-like sheet punching plate, a mesh braided with a
metal element wire and the like.
[0103] The particle size of the powder of negative electrode active
material affects the thickness and density of electrode, that is,
the capacity of electrode. The thickness of electrode is preferably
as thin as possible. This is because a smaller thickness of
electrode can increase the total surface area of the negative
electrode active material included in a battery. Therefore, an
average particle size of the powder of negative electrode active
material is preferably not more than 100 .mu.m. As the average
particle size of the powder of negative electrode active material
decreases, the reaction area of the powder increases, thereby
resulting in excellent rate characteristics. However, when the
average particle size of the powder of negative electrode active
material is too small, the properties and condition of the surface
of the powder change due to oxidation etc. so that it becomes
difficult for lithium ions to enter into the powder. In such a
case, the rate characteristics and the efficiency of
charging/discharging may decline over time. Therefore, the average
particle size of the powder of negative electrode active material
is preferably 0.1 to 100 .mu.m, and more preferably 1 to 50
.mu.m.
<Production Method of Battery>
[0104] A nonaqueous electrolyte secondary battery according to the
present embodiment includes the negative electrode as described
above, a positive electrode, a separator, and an electrolytic
solution or electrolyte. The shape of the battery may be a
cylindrical type, a square shape as well as a coin type and a sheet
type. The battery of the present embodiment may be a battery
utilizing a solid electrolyte such as a polymer battery and the
like.
[0105] The positive electrode of the battery of the present
embodiment preferably contains a transition metal compound
containing a metal ion as the active material. More preferably, the
positive electrode contains a lithium (Li)-containing transition
metal compound as the active material. An example of the
Li-containing transition metal compound is LiM.sub.1-xM'xO.sub.2,
or LiM.sub.2yM'O.sub.4. Where, in the chemical formulae,
0.ltoreq.x, y.ltoreq.1, and M and M' are respectively at least one
kind of barium (Ba), cobalt (Co), nickel (Ni), manganese (Mn),
chromium (Cr), titanium (Ti), vanadium (V), iron (Fe), zinc (Zn),
aluminum (Al), indium (In), tin (Sn), scandium (Sc) and yttrium
(Y).
[0106] However, the battery of the present embodiment may use other
positive electrode materials such as transition metal
chalcogenides; vanadium oxide and lithium (Li) compound thereof;
niobium oxide and lithium compound thereof; conjugated polymers
using organic conductive substance; Shepureru phase compound;
activated carbon; activated carbon fiber; and the like.
[0107] The electrolytic solution of the battery of the present
embodiment is generally a nonaqueous electrolytic solution in which
lithium salt as the supporting electrolyte is dissolved into an
organic solvent. Examples of lithium salt include LiClO.sub.4,
LiBF.sub.4, LiPF.sub.6, LiAsF.sub.6, LiB(C.sub.6H.sub.5),
LiCF.sub.3SO.sub.3, LiCH.sub.3SO.sub.3,
Li(CF.sub.3SO.sub.2).sub.2N, LiC.sub.4F.sub.9SO.sub.3,
Li(CF.sub.2SO.sub.2).sub.2, LiCl, LiBr, and LiI. These may be used
singly or in combination. The organic solvent is preferably
carbonic ester, such as propylene carbonate, ethylene carbonate,
ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate.
However, other various kinds of organic solvents including
carboxylate ester and ether are usable. These organic solvents may
be used singly or in combination.
[0108] The separator is placed between the positive electrode and
the negative electrode. The separator serves as an insulator.
Further, the separator greatly contributes to the retention of
electrolyte. The battery of the present embodiment may include a
well known separator. The separator is made of, for example,
polypropylene or polyethylene, which is polyolefin-based material,
or mixed fabric of the two, or a porous body such as a glass
filter. The above described negative electrode, positive electrode,
separator, and electrolytic solution or electrolyte are
accommodated in a container to produce a battery.
[0109] Hereinafter, the negative electrode active material, the
negative electrode, and the battery of the present embodiment
described above will be described in more detail by using Examples.
It is noted that the negative electrode active material, the
negative electrode, and the battery of the present embodiment will
not be limited to Examples shown below.
EXAMPLES
[0110] Powdered negative electrode active materials, negative
electrodes, and coin batteries of Inventive Examples 1 to 5 of the
present invention and Comparative Examples 1 to 3 shown in Table 1
were produced by the following method. Then, changes in the crystal
structure of each negative electrode active material caused by
charging/discharging were confirmed. Further, discharge capacity
(discharge capacity per volume) and cycle characteristics of each
battery were investigated.
TABLE-US-00001 TABLE 1 Production conditions (Conditions for
adjusting oxide layer thickness and oxygen concentration in oxide
layer) Atomic Oxygen Oxygen con- Temperature Thickness
concentration Battery characteristics quantity in centration in of
of surface of oxygen Initial Discharge Capacity melting
pulverization pulverization oxide in surface discharge capacity at
retention Chemical atmosphere atmosphere atmosphere layer oxide
layer capacity 20th cycle ratio composition (vol-ppm) (vol %)
(.degree. C.) (nm) (%) (mAh/cm.sup.3) (mAh/cm.sup.3) (%) Inventive
(1) Cu-23at%Sn- 500 0.1 20 1.3 22.5 2078 2032 97.8 Example 1 5at%Si
Inventive (1) Cu-23at%Sn- 500 1.0 20 1.5 24.4 2220 2172 97.8
Example 2 5at%Si Inventive (1) Cu-23at%Sn- 500 4.0 20 2.0 42.7 1917
1796 93.7 Example 3 5at%Si Comparative (1) Cu-23at%Sn- 500 21.0 40
12.0 72.1 1624 1246 76.7 Example 1 5at%Si Inventive (2) Cu-20at%Sn-
500 0.1 20 1.1 21.8 2145 2063 96.2 Example 4 10at%Al Comparative
(2) Cu-20at%Sn- 500 21.0 40 12.9 73.4 1549 1178 76.0 Example 2
10at%Al Inventive (3) Cu-25at%Sn 500 0.1 20 0.9 20.6 2089 1956 93.6
Example 5 Comparative (3) Cu-25at%Sn 500 21.0 40 13.4 74.6 1479
1026 69.4 Example 3
Inventive Example 1 of the Present Invention
[Production of Negative Electrode Active Material]
[0111] A mixture of a plurality of starting materials (elements)
was high-frequency melted in a nozzle made of boron nitride in
argon gas atmosphere to produce molten metal such that the final
chemical composition of the negative electrode active material
becomes the chemical composition listed in the "chemical
composition" column in Table 1. The oxygen concentration in the
argon gas atmosphere was as listed in Table 1.
[0112] A rapidly solidified foil band was produced by spraying the
molten metal onto a rotating copper roll. The thickness of the foil
band was 20 to 40 .mu.m. This foil band was pulverized by a
grinding machine (automatic mortar) into alloy powder of a size of
not more than 45 .mu.m. At this moment, the automatic mortar was
put into a glove box and the oxygen concentration in the
pulverization atmosphere, and the atmospheric temperature were
adjusted as shown in Table 1. This alloy powder was used as the
negative electrode active material. The final chemical composition
of this negative electrode active material was as listed in the
"chemical composition" column in Table 1. To be specific, the
chemical composition of the alloy powder contained 23 at % of Sn
and 5 at % of Si, with the balance being Cu.
[Production of Negative Electrode]
[0113] The above powdered negative electrode active material,
acetylene black (AB) as a conductive assistant, styrene-butadiene
rubber (SBR) as a binder (2-fold dilution), and
carboxymethylcellulose (CMC) as a thickening agent were mixed in a
mass ratio of 75:15:10:5 (blending quantity was 1 g:0.2 g:0.134
g:0.067 g). Then, a kneading machine was used to produce a negative
electrode compound slurry by adding distilled water to the mixture
such that slurry density was 27.2%. Since the styrene-butadiene
rubber was used by being diluted 2-fold with water, 0.134 g of
styrene-butadiene rubber was blended when weighing.
[0114] The produced negative electrode compound slurry was applied
on a metal foil by using an applicator (150 .mu.m). The metal foil
applied with the slurry was dried at 100.degree. C. for 20 minutes.
The metal foil after drying had a coating film made up of the
negative electrode active material on the surface. The metal foil
having the coating film was subjected to punching to produce a
disc-shaped metal foil having a diameter of 13 mm. The metal foil
after punching was pressed at a press pressure of 500 kgf/cm.sup.2
to produce a plate-shaped negative electrode material. In negative
electrodes to be used for the evaluation and measurement of the
negative electrode active material other than the determination of
crystal structure, the metal foil was copper foil. In a negative
electrode to be used for the determination of crystal structure,
the metal foil was nickel foil.
[Production of Battery]
[0115] The produced negative electrode, EC-DMC-EMC-VC-FEC as the
electrolytic solution, a polyolefin separator (.phi.7 mm) as the
separator, and a metal Li plate (.phi.19.times.1 mmt) as the
positive electrode material were prepared. Thus prepared negative
electrode, the electrolytic solution, the separator, and the
positive electrode were used to produce a coin battery of 2016
type. Assembly of the coin battery was performed within a glove box
in argon atmosphere.
[Measurement of Thickness of Surface Oxide Layer and Atomic
Concentration of Oxygen in Surface Oxide Layer]
[0116] As described above, the oxide layer refers to a layer having
a thickness corresponding to a half of the depth at which the
oxygen concentration becomes constant when the alloy powder is
subjected to quantitative analysis of oxygen from the surface in
the depth direction.
[0117] The thickness of surface oxide layer, and the atomic
concentration of oxygen in the surface oxide layer were determined
by creating a depth direction profile of oxygen under the following
conditions by using a micro-Auger electron spectroscopic analysis
apparatus.
[0118] Apparatus: Model 680 manufactured by ULVAC-PHI, Inc.
[0119] Primary beam: Acceleration voltage of 10 kV, and sample
current of 10 nA
[0120] Sputtering: Ar ion, acceleration voltage of 3 kV, sputtering
speed of 1.1 nm/min (SiO.sub.2 conversion)
[0121] A conversion value from Argon sputtering rate of SiO.sub.2
was used for determining the distance in the depth direction.
Moreover, for the determination of oxygen concentration, Auger peak
intensities of energy corresponding to each of the constituting
elements of the negative electrode material, and oxygen were
measured in the depth direction, and converted into atomic %. As a
result of this, values of the oxygen concentration of the surface,
and the thickness of the above described oxide layer were obtained,
respectively. In this occasion, although carbon is detected due to
contamination of the surface, that part is treated as an excluded
number, and excluded when converting analysis values.
[Determination of Crystal Structure]
[0122] The crystal structures of the powdered negative electrode
active material before use for the negative electrode, the crystal
structures of the negative electrode active material in the
negative electrode before initial charging, and the crystal
structures of the negative electrode active material in the
negative electrode after one to 20 times of charging and
discharging were determined by the following method. X-ray
diffraction measurements were carried out for the target negative
electrode active materials to obtain measured data. Then, based on
the obtained measured data, crystal structures included in the
target negative electrode active materials were determined by
Rietveld method. More specifically, the crystal structures were
determined by the following method.
[0123] (1) Crystal Structure Analysis of Powdered Negative
Electrode Active Material Before Use in Negative Electrode
[0124] X-ray diffraction measurements were carried out for the
powder (not more than 45 .mu.m) of the negative electrode active
materials before use in the negative electrode to obtain measured
data of X-ray diffraction profile.
[0125] To be specific, SmartLab (product of Rigaku Co., Ltd) (rotor
target maximum output 9 KW; 45 kV-200 mA) was used to obtain X-ray
diffraction profiles of the powder of the negative electrode active
materials.
[0126] Based on the obtained X-ray diffraction profiles (measured
data: FIG. 3 (e)), crystal structures of alloy phases in the
negative electrode active material were analyzed by Rietveld
method.
[0127] The DO.sub.3 ordered structure is an ordered structure as
shown in FIG. 2A. In a Cu--Sn base alloy, mainly Cu is present at
atomic sites shown by a black circle and mainly Sn is present at
atomic sites shown by a white circle, in FIG. 2A. Respective sites
may be replaced by addition of a third element. It is known that
such a crystal structure falls into No. 225 (Fm-3m) of
International Table (Volume-A) in the classification of space group
representation. The lattice constant and atomic coordinates of this
space group number are as shown in Table 2.
TABLE-US-00002 TABLE 2 Parent phase (.beta..sub.1 Phase), Crystal
Structure: DO.sub.3 Space Group Number (International Table A): No.
225 (Fm-3m) Lattice Constant: a = 6.05 .ANG. Multiplicity/ Atomic
Wyckoff Atomic Coordinates Site Name Species Symbol x y z Sn1 Sn 4a
0.0 0.0 0.0 Cu1 Cu 8c 1/4 1/4 1/4 Cu2 Cu 4b 1/2 1/2 1/2
[0128] Accordingly, with the structure model of this space group
number being as the initial structure model of Rietveld analysis, a
calculated value of diffraction profile (hereinafter, referred to
as a calculated profile) of .beta..sub.1 phase (DO.sub.3 structure)
of this chemical composition was found by Rietveld method.
RIETAN-FP (program name) was used for Rietveld analysis.
[0129] Further, a calculated profile of the crystal structure of
.gamma..sub.1' phase was found as well. The crystal structure of
.gamma..sub.1' was 2H structure in the notation of Ramsdell symbol,
and the space group was No. 59-2 (Pmmn) of International Table
(Volume-A). The lattice constant and atomic coordinates of No. 59-2
(Pmmn) are shown in Table 3.
TABLE-US-00003 TABLE 3 M Phase (.gamma..sub.1' Phase), Crystal
Structure: 2H Space Group Number (International Table A): No. 59-2
(Pmmn) Lattice Constants: a = 4.379 .ANG., b = 5.498 .ANG., c =
4.615 .ANG. Multiplicity/ Atomic Wyckoff Atomic Coordinates Site
Name Species Symbol x y z Sn1 Sn 2b 1/4 3/4 1/6 Cu1 Cu 2a 1/4 1/4
1/6 Cu2 Cu 4e 1/4 0.0 2/3
[0130] A calculated profile was found by using RIETAN-FP supposing
that the crystal structure of the space group number of the above
describe Table 3 be the initial structure model of Rietveld
analysis. In FIG. 3, (d) shows a calculated profile of DO.sub.3
structure, and (c) shows a calculated profile of 2H structure.
[0131] A result of the analysis revealed that a .beta..sub.1 phase
(DO.sub.3 structure) which is the matrix phase of a .gamma..sub.1'
phase (2H structure) which is a kind of M phase, is present singly
in the negative electrode active material of Inventive Example
1.
[0132] (2) Crystal Structure Analysis of Negative Electrode Active
Material in Negative Electrode
[0133] The crystal structure of a negative electrode active
material in a negative electrode before charging was also
determined by the same method as that in (1). A measured X-ray
diffraction profile was measured by the following method.
[0134] The above described coin battery, which was before being
charged, was disassembled within the glove box in argon atmosphere,
and a plate-shaped negative electrode (including nickel foil) was
taken out from the coin battery. The negative electrode taken out
was enclosed in Myler foil (manufactured by DuPont). Thereafter,
the perimeter of the Myler foil was sealed by a thermocompression
bonding machine. Then, the negative electrode sealed by the Myler
foil was taken out of the glove box.
[0135] Next, a measurement sample was fabricated by bonding the
negative electrode to a reflection-free sample plate manufactured
by Rigaku Co., Ltd. (a plate of a silicon single crystal which was
cut out such that a specific crystal plane was in parallel with the
measurement plane) with a hair spray.
[0136] The measurement sample was mounted onto the X-ray
diffraction apparatus described below in (4), and the X-ray
diffraction measurement of the measurement sample was performed
under measurement conditions described below in (4).
[0137] (3) Analysis of Crystal Structure of Negative Electrode
Active Material in Negative Electrode after Charging and after
Discharging
[0138] The crystal structure of the negative electrode active
material in the negative electrode after one to 20 times of
charging and after one to 20 times of discharging was also
determined by the same method as that in (1). Measured X-ray
diffraction profiles were measured by the following method.
[0139] The above described coin battery was fully charged in a
charging/discharging test apparatus. The fully charged coin battery
was disassembled in the glove box, and a measurement sample was
fabricated by the same method as that in (2). The measurement
sample was mounted onto the X-ray diffraction apparatus described
below in (4), and X-ray diffraction measurement of the measurement
sample was performed under measurement conditions described below
in (4).
[0140] Moreover, the above described coin battery was fully
discharged. The fully discharged coin battery was disassembled in
the glove box, and a measurement sample was fabricated by the same
method as in (3). The measurement sample was mounted onto the X-ray
diffraction apparatus described below in (4), and X-ray diffraction
measurement of the measurement sample was performed at measurement
conditions described below in (4).
[0141] For a negative electrode which had been subjected to
charging and discharging repeatedly in a coin battery, X-ray
diffraction measurement was performed by the same method.
[0142] (4) X-Ray Diffraction Apparatus and Measurement Conditions
[0143] Apparatus: SmartLab manufactured by Rigaku Co., Ltd. [0144]
X-ray tube: Cu-K.alpha. ray [0145] X-ray output: 45 kV, 200 mA
[0146] Incident monochrometer: Johannson-type crystal (which
filters out Cu-K.alpha..sub.2 ray and Cu-Ku ray) [0147] Optical
system: Bragg-Brentano geometry [0148] Incident parallel slit: 5.0
degrees [0149] Incident slit: 1/2 degree [0150] Length limiting
slit: 10.0 mm [0151] Receiving slit 1: 8.0 mm [0152] Receiving slit
2: 13.0 mm [0153] Receiving parallel slit: 5.0 degrees [0154]
Goniometer: SmartLab goniometer [0155] X-ray source--mirror
distance: 90.0 mm [0156] X-ray source--selection slit distance:
114.0 mm [0157] X-ray source--sample distance: 300.0 mm [0158]
Sample--receiving slit 1 distance: 187.0 mm [0159]
Sample--receiving slit 2 distance: 300.0 mm [0160] Receiving slit
1--receiving slit 2 distance: 113.0 mm [0161] Sample--detector
distance: 331.0 mm [0162] Detector D/Tex Ultra [0163] Scan range:
10 to 120 degrees or 10 to 90 degrees [0164] Scan step: 0.02
degrees [0165] Scan mode: Continuous scan [0166] Scanning speed: 2
degrees/min or 2.5 degrees/min
[0167] (5) Analysis Results of X-Ray Diffraction Measurement
Data
[0168] X-ray diffraction data obtained in (1) and (3) are shown in
FIG. 3. In FIG. 3, (e) is an X-ray diffraction profile of powder of
a negative electrode active material, which was found in (1). In
the figure, (f) is an X-ray diffraction profile of the negative
electrode active material after first charging; and (g) is an X-ray
diffraction profile after first discharging. Reference symbol (a)
in FIG. 3 indicates a measured X-ray diffraction profile in which
similar X-ray diffraction is performed on a Myler film alone.
Reference symbol (b) in FIG. 3 indicates an X-ray diffraction
profile of Ni, which is calculated to identify diffraction lines of
Ni foil used for the current collector. Reference symbol (c) in
FIG. 3 indicates a calculated profile of 2H structure in the
chemical composition of the present example, and reference symbol
(d) in FIG. 3 indicates a calculated profile of DO.sub.3 structure
in the chemical composition of the present example.
[0169] (5-1)
[0170] From the X-ray diffraction data obtained in (1) and (2), it
can be confirmed that no significant reaction has occurred between
the negative electrode active material and the electrolytic
solution.
[0171] (5-2)
[0172] X-ray diffraction profiles of the "negative electrode active
material after charging" (FIG. 3(e)) and the "negative electrode
active material after discharging" (FIG. 3(f)) were compared with
each other. As a result, the diffraction line repeatedly changed in
a reversible manner at a position where the diffraction angle
2.theta. is near 43.3.degree. (position caused by M phase
(.gamma..sub.1' phase))(hereinafter, referred to as a strongest
diffraction line of M phase). That is, a structural change was
exhibited.
[0173] (5-3)
[0174] Accordingly, the crystal structures of the "negative
electrode active material after charging" and the "negative
electrode active materials after discharging" were determined by
using Rietveld method.
[0175] For example, in the negative electrode active material, the
crystal plane A shown in FIG. 2D and the crystal plane B shown in
FIG. 2C are alternately layered in the DO.sub.3 structure of the
matrix phase shown in FIGS. 1 and 2A. When a phase transformation
occurs between the DO.sub.3 structure and .gamma..sub.1' phase
which is a kind of M phase, as shown in FIGS. 2A and 2B, the
crystal plane B regularly undergoes shuffling due to shear stress,
thereby being displaced to the position of crystal plane B'. In
this case, phase transformation (M transformation) occurs without
diffusion of the host lattice. In the 2H structure after M
transformation, the crystal plane A shown in FIG. 2D and the
crystal plane B' shown in FIG. 2E are alternately layered.
[0176] Then, it was judged whether the crystal structure of the
negative electrode active material in the negative electrode of the
present example involved M transformation or was not accompanied
thereby (that is, involved diffusion of host lattice at the time of
charging/discharging) by comparing the measured data of the X-ray
diffraction profiles of the negative electrode active material
after charging and after discharging, the calculated profile of
.beta..sub.1 phase (DO.sub.3 structure) (FIG. 3(d): 2.theta. angle
positions of representative diffraction lines are denoted by (black
circle) symbols), and the calculated profile of .gamma..sub.1'
phase (2H structure) (FIG. 3(c): 2.theta. angle positions of
representative diffraction lines are denoted by .box-solid. (black
square) symbols).
[0177] Referring to FIG. 3 (f), in the X-ray diffraction profile,
the intensity of the strongest diffraction line of M phase of near
43.5.degree. increased as a result of initial charging, and
decreased as a result of consecutive discharging. It can be judged
that this diffraction line resulted from the formation of M phase
(.gamma..sub.1') by M transformation, as will be next described,
from calculated profiles of RIETAN-FP).
[0178] To be specific, as shown in (f), in the 2H structure, there
were increases in some of the peak intensities of 2.theta. angle
positions (.box-solid.(black square) symbol) corresponding to the
2H structure of FIG. 3(c) including the strongest diffraction line
of M phase at 43.5.degree. in the X-ray diffraction profile after
the first charging. On the other hand, there were decreases in some
of the peak intensities of .box-solid. (black circle) symbol
corresponding to DO.sub.3 structure. In particular, intensity peaks
near 43.5.degree. did not appear in the X-ray profile (simulation
result) of any crystal structure other than 2H. For X-ray
diffraction of a constituting member other than active material,
which appears during measurement, a measured diffraction profile of
a Myler foil and a calculated profile of nickel of the current
collector are shown by (a) and (b) in FIG. 3 respectively, in which
2.theta. angle positions of major diffraction lines are indicated
by .diamond. symbols and .DELTA. symbols, respectively. Since
diffraction lines resulted from these members appear in the
measured profiles of (f) and (g) in FIG. 3, corresponding positions
are indicated by .diamond. (white rhombus) symbols and A (white
triangle) symbols.
[0179] From the above, the negative electrode of the present
Example contained an alloy phase which underwent M transformation
to become M phase (2H structure) as a result of charging, and
became a matrix phase (DO.sub.3 structure) as a result of
discharging. That is, the negative electrode of the present Example
contained an alloy phase which underwent M transformation when
occluding lithium ions which are metal ions, and underwent reverse
transformation when releasing lithium ions.
[0180] In the negative electrode of the present Example, M
transformation at the time of charging, and reverse transformation
at the time of discharging were repeated.
[0181] The full width at half maximum of a diffraction line
decreased along with charge-discharge cycles. From this, it is
considered that occlusion and release of lithium ions relaxed
strain of the negative electrode active material.
[Charge-Discharge Performance Evaluation of Coin Battery]
[0182] Next, discharge capacity and cycle characteristics of the
battery of Inventive Example 1 were evaluated.
[0183] Constant current doping (corresponding to the insertion of
lithium ions into electrode, and the charging of lithium ion
secondary battery) was performed to a coin battery at a current
value of 0.1 mA (a current value of 0.075 mA/cm.sup.2) or a current
value of 1.0 mA (a current value of 0.75 mA/cm.sup.2) until the
potential difference against the counter electrode becomes 0.005 V.
Thereafter, doping capacity was measured by continuing doping
against the counter electrode at a constant voltage until the
current value became 7.5 .mu.A/cm.sup.2 while retaining 0.005
V.
[0184] Next, de-doping capacity was measured by performing
de-doping (which corresponds to desorption of lithium ions from the
electrode, and discharge of the lithium ion secondary battery) at a
current value of 0.1 mA (a current value of 0.075 mA/cm.sup.2) or a
current value of 1.0 mA (a current value of 0.75 mA/cm.sup.2) until
the potential difference becomes 1.2 V.
[0185] The doping capacity and de-doping capacity correspond to
charge capacity and discharge capacity when the electrode is used
as the negative electrode of the lithium ion secondary battery.
Therefore, the measured dope capacity was defined as the charge
capacity, and a measured de-doping capacity was defined as the
discharge capacity.
[0186] Charging and discharging were repeated 20 times at the same
conditions as described above. Then, a capacity retention ratio (%)
was defined as "the discharge capacity at the time of de-doping of
the 20th cycle" divided by "the discharge capacity at the time of
de-doping of the 1st cycle, and multiplied by 100. In the coin
battery of Inventive Example 1, the initial discharge capacity, the
discharge capacity of the 20th cycle, and the capacity retention
ratio were as listed in Table 1.
Inventive Examples 2 and 3, and Comparative Example 1
[0187] In Inventive Examples 2 and 3, negative electrode active
materials, negative electrodes, and coin batteries were produced in
the same way as in Inventive Example 1 excepting that the oxygen
concentrations in the pulverization atmosphere were changed to the
concentration shown in Table 1. In Comparative Example 1, the
negative electrode active material, the negative electrode, and the
coin battery were produced in the same way as in Inventive Example
1 excepting that the oxygen concentration in the pulverization
atmosphere and the temperature of the pulverization atmosphere were
changed as shown in Table 1.
[0188] Determination of crystal structure, and evaluation of oxygen
concentration in the negative electrode active material and various
charge-discharge performances of the coin battery were performed in
the same way as in Inventive Example 1.
[0189] The result of the determination of crystal structure was the
same as in Inventive Example 1. That is, it was confirmed that the
alloy phases of Inventive Examples 2 and 3, and Comparative Example
1 had a crystal structure that undergoes M transformation when
occluding lithium ions, and undergoes reverse transformation when
releasing lithium ions.
[0190] The results of the evaluation of the thickness of the
surface oxide layer of the negative electrode active material, the
oxygen concentration in the surface oxide layer, and various
charge-discharge performances of the coin battery were as shown in
Table 1.
Inventive Example 4 and Comparative Example 2
[0191] In Inventive Example 4, a negative electrode active
material, negative electrode, and coin battery were produced in the
same way as in Inventive Example 1 excepting that the final
chemical composition of the negative electrode active material was
changed to the composition according to Table 1. In Comparative
Example 2, the negative electrode active material, negative
electrode, and coin battery were produced in the same way as in
Inventive Example 4 excepting that the oxygen concentration in the
pulverization atmosphere and the temperature of the pulverization
atmosphere were changed as shown in Table 1.
[0192] Determination of crystal structure, and evaluation of oxygen
concentration in the negative electrode active material, and
various charge-discharge performances of the coin battery were
performed in the same way as in Inventive Example 1.
[0193] The result of the determination of crystal structure was the
same as in Inventive Example 1. That is, it was confirmed that the
alloy phases of Inventive Examples 2 and 3, and Comparative Example
1 had a crystal structure that undergoes M transformation when
occluding lithium ions, and undergoes reverse transformation when
releasing lithium ions.
[0194] Results of the evaluation of the thickness of the surface
oxide layer of the negative electrode active material, the oxygen
concentration in the surface oxide layer, and various
charge-discharge performances of the coin battery were as shown in
Table 1.
Inventive Example 5 and Comparative Example 3
[0195] In Inventive Example 5, the negative electrode active
material, negative electrode, and coin battery were produced in the
same way as in Inventive Example 1 excepting that the final
chemical composition of the negative electrode active material was
changed to the composition listed in Table 1. In Comparative
Examples 3, the negative electrode active material, negative
electrode, and coin battery were produced in the same way as in
Inventive Example 5 excepting that the oxygen concentration in the
pulverization atmosphere and the temperature of the pulverization
atmosphere were changed as shown in Table 1.
[0196] Determination of crystal structure, and evaluation of oxygen
concentration in the negative electrode active material and various
charge-discharge performances of the coin battery were performed in
the same way as in Inventive Example 1.
[0197] According to the determination of crystal structure, the
alloy phase changed as follows as charging/discharging was
performed. The alloy phase was M phase (2H structure) before
initial charging, M phase (2H structure) after initial charging,
and a matrix phase (DO.sub.3 structure) after initial discharging,
and thereafter, the alloy phase underwent M transformation to be
transformed into M phase (2H structure) through charging, and
became matrix phase (DO.sub.3 structure) through discharging. That
is, it was confirmed that the negative electrode of the inventive
example had an alloy phase that undergoes M transformation when
occluding lithium ions which are metal ions, and undergoes reverse
transformation when releasing lithium ions.
[0198] Results of evaluating the thickness of the surface oxide
layer of the negative electrode active material, the oxygen
concentration in the surface oxide layer, and various
charge-discharge performances of the coin battery were as shown in
Table 1.
[0199] Referring to Table 1, in Inventive Examples 1 to 5, in the
production process of molten metal, the oxygen concentration in the
atmospheric gas wad not more than 5000 ppm. Further, the oxygen
concentration in the pulverization process was not more than 20 vol
%. For that reason, the thickness of the oxide layer on the surface
of the powder material was not more than 10 nm, and the oxygen
content in the oxide layer was not more than 70 at %. As a result
of that, discharge capacities at the initial and the 20th cycles
were not less than 1700 mAh/cm.sup.3, and the capacity retention
ratio was not less than 90%.
[0200] On the other hand, in Comparative Examples 1 to 3, the
oxygen concentration in the pulverization process was more than 20
vol %. For that reason, the thickness of the oxide layer of the
powder material was more than 10 nm, and the oxygen content in the
oxide layer was more than 70 at % as well. As a result of that, the
discharge capacities at the initial and the 20th cycles were less
than 1700 mAh/cm.sup.3, and the capacity retention ratio was less
than 90%.
[0201] So far, embodiments of the present invention have been
described. However, the above described embodiments are merely
examples to carry out the present invention. Therefore, the present
invention will not be limited to the above described embodiments,
and can be carried out by appropriately modifying the above
described embodiments within a range not departing from the spirit
thereof.
* * * * *